Opposition leader and Zionist Union head Isaac Herzog was questioned under caution on Sunday on suspicion of receiving unlawful campaign donations.

Police are investigating whether Herzog received unlawful donations, failed to report donations and provided a false statement during the 2013 Labor Party primary campaign. The questioning received the consent of Attorney General Avichai Mendelblit.

Speaking on Sunday evening, Herzog said that "it's no secret that over the past two weeks I declared again and again that I will come at any time and to any place and give my version of events. I did it this morning, and if need be I will go and help as much as it takes so this affair will be quickly behind us. I presented myself this morning without delay, I trust the law enforcement agencies and am grateful for their fair and respectful conduct.”

"You know that it's no secret that any time the issue of general elections or internal elections comes up various strange complaints surface, and it's good that things are being checked, as they were checked in the past with previous opposition heads that were investigated by police," he said at a toast for party members.

"Believe me, I'm calm, and if there's someone in the party who isn't calm – they better calm down. A leader's role is to face such phenomena, overcome them, carry on, lead and win," he added.

"I've heard many party members and other party leaders and I appreciate the complete backing that I received from them. This is the essence of political partnership. This isn't easy, of course, but what doesn’t kill you makes you stronger, and this is the time to be together."

Earlier, upon leaving the investigation, Herzog addressed the incident in a text message to MKs, saying that "I'm completely convinced that my actions remain untarnished. I thank you for your friendship and backing." In response, he received a slew of supportive messages from Labor colleagues such as MKs Yoel Hasson, Erel Margalit and Nachman Shai.

Over the past few months several people have been summoned for questioning so that police could ascertain whether Herzog may have violated the law in the way he funded his primary campaign, which ended with his defeat of then-party chairman Shelly Yacimovich. Since the examination became public, police have questioned several Herzog associates, some of them under caution, which essentially turned what had been a preliminary examination into a criminal investigation.

Yacimovich, whom Herzog defeated in 2013, responded to the news of Herzog's questioning, saying that she is "convinced that Herzog has the best interests of the Party and the opposition in mind and I will work together with him and party members to decide what steps should be taken.

"No doubt, an investigation under caution of the party leader and opposition leader worsens the situation. I completely trust the police and legal authorities," she concluded.

MK Merav Michaeli, the Zionist Union's whip, expressed her support for the party chairman.

"Herzog said that he's calm and I believe him and am sure that he'll be able to explain his actions. As he promised that he would do from the moment that the investigation became known, he has shared the information he knows and gave his version to the law enforcement agencies. I hope that justice will be served as quickly as possible and that this will shortly be behind him and behind us."

Meretz chairwoman Zehava Galon also responded to the incident. "I wish MK Isaac Herzog that he'll manage to disprove the claims against him and prove his innocence. If the investigation develops into an indictment, Herzog will have to suspend himself from the position of opposition leader.

"In the meanwhile, we need to let the law enforcement authorities do their work. The role of opposition leader is a stately one equivalent to that of a minister, and therefore it's appropriate that the High Court rule that says that a minister who is indicted must quit will also apply to whoever serves in this role."

Sunday's investigation wasn’t a first for Herzog. In 1999, he was summoned for questioning as part of an investigation against former Prime Minister Ehud Barak's associates regarding party funding violations. Herzog, who was cabinet secretary at the time, chose to remain silent during that investigation.

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